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Description

Solution

IDCODE looping is a function that performs many iterations of the JTAG operation "Get Device ID". You can set the number of iterations performed (the default number is 10000).

The IDCODE looping function also shifts in an explicit IDCODE command. Issuing the IDCODE command utilizes TDI to shift commands into the device. This is different from initializing the JTAG chain, which only shifts out the IDCODE register loaded by default into the Data Register after power up. This operation is not dependent on data coming in to TDI.

So, if the chain initializes but IDCODE looping fails, there might be a problem with TDI.

In general, failing JTAG operations can be due to problems with signal integrity or power.